Simultaneous use of a number of liquid storage vessels or tanks having their outlets connected in parallel to a common feed or drain pipe is well known in the art. Certain problems occur when this type of piping arrangement is used. A sample of these problems is the connecting of propellant tanks in a space craft wherein the propellant is fed from the outlet of each tank to a common feed line for the main propulsion rocket engines or attitude control engines. A major problem arises when the parallel propellant tanks drain unevenly which can be caused from a number of factors or conditions. Examples of these conditions could be that the tanks were not equally filled prior to use or that the outlet piping from each of the tanks has different flow restrictions and thus, different flow rates. It is quite important when dealing with propellants that each of the tanks be drained equally to the lowest possible level without introducing air or pressurization gas into the propellant outlet feed system. Once gas is introduced into the common outlet pipe the liquid flow from all remaining tanks is stopped. Thus, if gas is prematurely introduced into the propellant feed line, the propellant flow wculd be interrupted causing the engine to shut down prematurely preventing full usage of all propellants available with possible mission failure.
In prior tank systems it was common practice to estimate the total liquid level quantities within the tanks. In most cases it is possible to estimate within 5-10% of the actual tank liquid volume. The liquid load would then be increased by the estimated potential error so that early depletion of one tank prior to the others would not occur. In some cases the installation of isolation valves such as check valves can prevent transfer between tanks and calibration of the flow losses of the liquid outlet system can help to equalize the actual liquid flow rate within the system. These solutions, however, are unnecessary when the arrangement according to the present invention is utilized.
Throughout this application although direct reference may be made to space craft propellant feed systems, it is to be understood that this invention can be applied to any liquid supply system having parallel multiple tanks that must provide gas-free liquid and continuous flow reliability.